Overgrowth will definitely have a GNU/Linux client and although Natural Selection 2 GNU/Linux client hasn’t yet confirmed there is a high chance – and it will be much higher if you’ll preorder now.
In ANY case you will get the GNU/Linux, MacOS and Windows clients for both games (if NS2 will have one – that is…).
You can always post at NS2 forums and try to make them commit to a GNU/Linux client before preordering (if you do this quick, you only got 6 days) , there is also a huge NS2 GNU/Linux thread, so I would say that the chances are pretty high.

Overgrowth
Wolfire is an independent team of four guys, working non-stop to create the best ninja rabbit game possible: Overgrowth. Instead of licensing someone else’s engine, we’ve built our own from scratch, so that we can custom tailor every feature to support our innovative, combat-based gameplay. Please click on the screenshots to scope out how it looks so far. You can see more in-depth demonstrations of our technology on Wolfire’s YouTube channel.
Desert Fort
Overgrowth takes place in the savage world of Lugaru where rabbits, wolves and other animals are forced to use paws, claws and medieval weaponry to engage each other in battle. Overgrowth will be a seamless integration of brutal close quarters combat and fluid platforming. Powered by our brand new Phoenix Engine, Overgrowth will have cutting edge graphics, brutal physics, realistic AI and intuitive combat controls that will immerse players and tap into their primal survival instincts.
Editor UI
We’ve spent the past year building the core of the Phoenix Engine. We’ve added a lot of slick features like our off-the-grid optimized terrain with normal-mapped detail textures, direct lighting, ambient occlusion, realistic lens flares, volumetric haze, innovative plant shading and rendering techniques. We’ve also thrown in a complete editor suite to go with it. Our integrated map editor, decal editor, sky editor, rigging editor and animation editor make modding a breeze. Combat is coming soon.

Natural Selection 2
Natural Selection 2 is the follow-up to the most popular independent mod for Half-life which won Gamespy’s Mod of the Year. It is a multiplayer first-person shooter with unique real-time strategy elements and is being developed on its own custom engine called Spark.
The game features truly unique sides (Marines vs. Aliens), dynamic environments (including alien “infestation” that grows and deforms environments during the course of the game) and real-time strategy (some players can choose to play from the top-down as Commanders!).
Lighting in-editor
It will also be the most moddable game ever released and will include most of the game’s source code (in Lua script) along with all the programming and art tools we are using to build the game. We expect the mod scene to explode and offer unlimited variations, mods and entirely new games. We will also release many updates to keep the core game fresh with new maps, weapons and abilities.
Alpha environment
Unknown Worlds is a small company dedicated to bringing you the best games possible. We are known for working closely with our community and like to release our tools as soon as they are ready for feedback. We recently released our editor and environment art to all our pre-orderers and are looking forward to releasing our other tools and our alpha to you as well!
Your pre-order of Natural Selection 2 allows us to remain independent and able to release the best game possible. We would not be here today without you and your support so we offer our deeply-felt thanks!

I like yourself though this way, that’s why I haven’t bought yet Torchlight or Venetica.
But this is for this pack I want to try a different approach, let me explain…

Overgrowth will already have a GNU/Linux client, so you already get what you pay for.
NS2 is mostly cross platform as their engine, there is a huge thread about people asking for a native GNU/Linux client in their forums : http://www.unknownworlds.com/ns2/forums/index.php?showtopic=104522 , in which the developers confirmed that it’s very possible.

In most times we the GNU/Linux gamers buy games that already have a GNU/Linux client, on other times we preorder s specific amount of games so they develop a GNU/Linux client (like it happens with most Deck13 games).
This time I propose to preorder (Overgrowth alone worth 90% of the price), and post in their forums that you have already preordered and you are a GNU/Linux user – you have supported them in their darkest hour when they had no money to continue the development , and in turn you want them to support you.

I think it will work as there is no reason why it won’t.

Also as you see Comprox, one of the NS2 developers has posted on LGN blog, so it’s a good sign.

Well, it’s very unlikely that Torchlight and Venetica will get a Linux client.
I’d bet on all the rabbit-ninjas in the world that we get a GNU/Linux client for Overgrowth. It’s virtually set in stone. As soon as Awesomnium is ported to Linux, Icculus will start his porting work.

I wouldn’t bet my pants on getting a Linux client for NS2, but I already ordered it at the first possibility in the hope of getting one and I still hope that the guys will port it. It’s likely that they’ll do it, but not 100% certain.